The factors for importance in processes for preparing pulp include:
(1) physical properties of the fiber which carry over to the paper product to give satisfactory strength. These are conventionally evaluated in terms of burst, tear, and breaking length. PA1 (2) freeness which is related to dewatering on conventional paper making equipment. PA1 (3) brightness--It is desirable for most purposes that the paper made from the pulp be white or at least light coloured. The greater the brightness of the pulp the less the cost of chemicals for bleaching. PA1 (4) yield--The higher the yield, the greater cost efficiency in the utilization of forest products. PA1 (5) chemical consumption--Chemicals are required for pulping processes but these are costly and it is therefore desirable to minimize chemical consumption and also to use chemicals which are available at a reasonable cost. PA1 (6) time of pulping. This also affects cost in that it involves the use of costly equipment and energy in terms of heat input to maintain the cooking temperature. PA1 (7) refining energy--The cost of the energy required for pulping processes that include mechanical refining is an important cost factor.
Chemical pulping leads to strong papers, but is costly in terms of low yield and high chemical consumption. There are also accompanying problems of pollution abatement.
Mechanical pulping provides good yields but the refining costs are high, especially in the case of thermo mechanical pulping (TMP) and refiner mechanical pulping (RMP) and the strength of paper produced is rather low. In the case of groundwood, even though the defibrating energy is low, the pulp and paper properties are so low that it can be used only in admixture with other pulps.
There has been increased interest in recent years in so-called chemimechanical or semi-chemical processes which provide pulps of a strength that is adequate for most purposes and in which the yield is of the order of 90% or more. The drawback is, however, the high power requirements for the mechanical refining part of the chemimechanical or semi-chemical process due to the high percentage of lignin and fiber stiffness. The chips are not as soft as those produced by chemical pulping.
An alternative to high energy mechanical refining in equipment such as a disc refiner, is to soften wood chips with steam under high pressure followed by explosive decompression. This was indeed the process invented by Mason in the 1920's and used for hardboard manufacture. Chips were steamed at low pressure for about one minute then at high pressure for two minutes, and then brought to an even higher pressure followed by discharge of superheated chips to atmospheric pressure to explode the chips into a pulp called gun stock which was then further refined. Although the pulp resulting from the Mason process had high freeness and bulk and although the step of explosive decompression resulted in a saving of the power needed for further refining, the physical strength, as evaluated in terms of burst, tear and breaking length, was low. The fibers were therefore unsuitable for papermaking. Another problems was the relatively dark colour which would have required excessive chemical consumption for bleaching. There was also considerable yield loss due to acidic hydrolytic degradation due to the wood acids liberated at the high temperature used.
According to Asplund Svensk Papperstid (1953) 56,550 pulp with good paper making properties can be produced by a process involving explosive decompression if the steam temperature is controlled to between 100.degree. C. and 160.degree. C. Higgins et al in Appita 32(3) 187-200 (November 1978) suggested that the Asplund process could be improved if the chips were chemically pretreated and the steam temperature was limited to less than 130.degree. C. In Higgins' modification of the Asplund process the pressure at a temperature of 130.degree. C. will be about 1.5 atmospheres.